1. Field of the Invention
This invention relates to thermistors and is particularly concerned with thermistor materials suitable for use over a medium temperature range lying between the range covered by normal thermistors and high temperature thermistors.
2. Description of the Prior Art
Thermistors are thermally-sensitive resistors. They may have either a positive or negative coefficient of resistance depending on such factors as composition and thermal treatment. Normal negative temperature coefficient (NTC) thermistors commercially available generally cover the temperature range -60.degree.C to 300.degree.C and high temperature NTC thermistors cover the range 600.degree.C to 1000.degree.C. These thermistors, however, do not usually possess practical resistance values or acceptable stability over the 300.degree.C to 600.degree.C temperature range. By practical resistance values is meant tens of ohms at one end of the range and hundreds of thousands of ohms at the other end of the range. Although some commercially available thermistors intended for use in the range -60.degree.C to 300.degree.C would have practical resistance values above 300.degree.C, their stability above 300.degree.C would not normally be commercially acceptable. The high temperature thermistor would have a resistance of the order of two megohms at around 600.degree.C which increases with decreasing temperature.
A previously known composition disclosed in British Pat. No. 874,882, utilizes a thermistor material formed from a mixture of zirconia and between 2% and 25% by weight of yttria, a specific embodiment containing 15% of yttria and 85% of zirconia. The use of praseodymium oxide in place of yttria was also suggested. By varying the percentage ratio, a minimum specific resistance is obtained at the preferred percentage ratio.